Syria told the UN climate talks in Bonn on Tuesday that it would join the Paris Agreement, leaving the United States as the only nation in the world opting to stay outside the landmark treaty.

"We are going to join the Paris Agreement," the Syrian delegate, speaking in Arabic, said during a plenary session at the 196-nation talks, according to Safa Al Jayoussi of the IndyAct NGO, who was monitoring the session.

The United States ratified the 2015 pact but US President Donald Trump announced earlier this year that he would pull out, saying the pact did not serve US interests.

"It is our understanding that the government of Syria announced today their intent to join the Paris Agreement," Nick Nuttall, the spokesman for the UN climate body, told AFP.

Nuttall identified the Syrian delegate as Wadah Katmawi, the deputy minister of the ministry of local administration and environment.

Syria must submit their "instruments of ratification" at the UN headquarters in New York before their adherence becomes official, he added.

According to the Syrian parliament website, a bill was passed on October 22 to ratify the Paris accord, "in accordance with the Syrian Constitution which stipulates the protection of the environment".

Other parties at the 12-day negotiations, tasked with elaborating and implementing the agreement, welcomed the news.

"Syria joining the Paris Agreement will be a good thing," South Africa's chief negotiator Maesela Kekana told AFP.

"That's great," said Chai Qimin, a climate negotiator from China's National Center for Climate Change Strategy and International Cooperation.

"They were the last party to the UN Convention to sign the Paris Agreement," he said, referring to 1992 UN Framework Convention on Climate Change (UNFCCC), the bedrock treaty for UN climate talks.

"That leaves only the one who announced their withdrawal," he added, in an oblique reference to the United States.

War-torn Syria would be the 197th country to sign on to the climate pact, which vows to hold global warming to "well below" two degrees Celsius (3.6 degrees Fahrenheit).

Openly isolated on the climate issue at G-7 and G-20 meetings earlier this year, the United States said it intended to withdraw "unless the president can identify terms that are more favourable to American businesses, workers, and taxpayers."

The Trump administration has not said what those terms might be.

"This means Trump is in splendid isolation -- no one wants to be in his company," said Alden Meyer, a veteran climate analyst at the Washington-based Union of Concerned Scientists.

"With Syria on board, now the entire world is resolutely committed to advancing climate action -– all save one country," said Paula Caballero, director for climate change at the World Resources Institute.

"This should make the Trump administration pause and reflect on their ill-advised announcement about withdrawing from the Paris Agreement."

Signed in the French capital in December 2015, the treaty entered into force on November 4, 2016.

According to the terms, no country can formally submit their intent to withdraw until three years after the date of entry into force.

It will then take one year to fully withdraw, meaning the earliest the United States could officially withdraw is the day after the 2020 election.

"When even Syria -- with all its problems -- can see the sense of a global climate agreement, it really shows how ideologically wedded to climate denialism the US Republican Party has become," said Mohamed Adow, International Climate Lead for Christian Aid.